The invention relates to the preparation of single- or multi-component metal oxide sols from aqueous metal salt solutions or a solution of a metal salt mixture by electrolysis.
Processes for the preparation of metal oxide sols have frequently been described in the literature. In most cases, the sols are prepared by preparing an aqueous solution of a metal salt, which is then converted into the sol state by, for example, hydrolysis, which can be effected by heating, and/or by acid peptization and/or by addition of a base. The disadvantage of these processes is that often precipitations take place instead of the sol formation, which is a great disadvantage in particular when expensive metal salts are used, for example organometallic complexes. Accordingly, the use of metal salts during salt preparation is frequently limited to low concentrations.
An improved process for the preparation of the colloids operates with ion exchangers. However, the disadvantage of this process is the limited exchanging capacity and the low reactor volume. After completion of each process, the ion exchanger needs to be replaced and regenerated, so that the process can only proceed batchwise.
Metal sols can also be obtained by electrodialysis, in which, however, sol formation and precipitations frequently compete with one another.
Electrolytic processes for the preparation of colloidal titanium salt and tin salt solutions have already been disclosed in SU 706468 and SU 929741 A and in Kolloidn. Zh. 43 (4 ), 192-5 and 8 12-16.
However, the preparation of a titanium oxide sol by electrolysis of a TiCl.sub.4 solution can only be achieved in the presence of a stabilizing additive, such as, for example, ZrCl.sub.4. During electrolysis of the pure metal salt solution, decompositions and precipitations during sol formation were increasingly observed. In the electrolytic processes described in the literature, electrolytic units comprising three chambers are used, which is complicated and requires the use of ion exchange membranes.
Multi-component metal oxide sols can be prepared in high purity and at low temperatures by hydrolysis of metal alkoxide mixtures. A disadvantage of this method is the very high price of the metal alkoxides. Accordingly, there was a need to find a simple and relatively uncomplicated process in which stable single- or multi-component metal oxide sols can be prepared in high yields without the addition of stabilizing additives.